This paper develops the relationships between proton reduction at the surface of metals, and hydrogen evolution or hydrogen diffusion into the metal. Equations relating the permeation rate to the proton reduction rate are developed in the case of adsorption-absorption mechanism, with Volmer-Tafel reactions. Analytical expressions are derived, and three distinct regimes are evidenced: i/ a thin membrane-low current domain, where all reduced protons enter into the metal and diffuses to the exit face (i.e. the permeation rate is equal to the faradaic reaction rate) ; ii/ a thin membrane and high current domain, where the permeation rate is proportional to the square root of the proton reduction rate, but is independent of the membrane thickness and, iii/ a thick membrane high current situation, where the permeation rate is still proportional to the square root of the proton reduction rate, and also inversely proportional to the membrane thickness. These permeation regimes and their analytical expressions are then used to examine results published in the literature for α-iron and low alloy steel in different charging environments. It can be shown that the transition between thick and thin membrane regimes and low-high charging conditions are strongly interrelated. It was also possible to establish that governing equations could be described with two main parameters, i.e. a critical membrane thickness and a critical current density. The former appears to depend mainly on the metal properties, while the latter is a direct measure of the ability of the charging media to promote hydrogen entry in the metal.